Clutches which are activated or energized by electromagnetic coils are common components in rotary power transmission systems, both in stationary applications and in motor vehicles. Such electromagnetic clutches may be broadly characterized by whether they provide on-off energy transfer or modulating energy transfer. In the case of the former, dog clutches which may include auxiliary synchronizing devices are utilized whereas in the latter, friction clutch packs having a plurality of interleaved friction plates or discs are utilized. In either case, an electromagnetic operator which translates or compresses components of the clutch upon energization activates the clutch and upon deenergization deactivates or relaxes the clutch.
One type of torque transfer devices of the electrically actuated clutch type proportionally transfer torque from an input shaft to an output shaft based on the amount of current applied to an electrical actuator (applied as a constant current level based on applied voltage, or at an average level through pulse width modulation of applied voltage or some other modulation scheme). Each design requires the application of a certain amount of current to the electrical actuator to cause the clutch to transfer a given value of torque.
There is a constant drive toward reducing electric current consumption of motor vehicle components, in order to increase gas mileage, among other advantages of such a reduction. Inter-active torque management systems have provided closed loop torque feedback systems that measure the amount of torque being applied, in order to determine the minimum amount of torque required. Applying the minimum amount of torque required helps reduce electric current consumption and also may reduce wear on components. Likewise, there is also a need to provide accurate torque transfer in order to affect vehicle dynamics in prescribed fashion that promotes increased vehicle stability. In order to provide this functionality one must know the amount of torque transferred to the wheels during this maneuver, hence the need for torque feedback.
Some torque transfer devices include an electrical clutch operator having a solenoid coil. A solenoid coil is a coil of wire that provides a magnetic force when a current is passed through it. A solenoid can create controlled magnetic fields, and therefore, solenoids are often used as electromagnets to generate linear forces. Accordingly, a solenoid includes a fixed stator and a moving armature. Solenoids can be used in a variety of applications, such as in actively controlled couplings and differentials. In these types of applications, it is common to apply a known input to the solenoid to receive a predictable output force. However, one factor that affects the force output of a solenoid is the air gap present between the stator and the armature. For a solenoid used in an all-wheel drive (AWD) coupling, it has been difficult to measure or monitor the air gap directly. Therefore, is difficult to predict the input required to obtain desired outputs.
Due to manufacturing and component variations between units of a given design, the actual current required to produce certain torque transfer will vary. In other words, for a given current, the amount of torque each unit will transfer is often different. Thus, typically, the exact calibration curve of the coupling device is unknown; in other words, the exact amount of current input required to achieve a desired torque output is not exactly known. Therefore, it must be estimated, and various means have been used to do so. Still, there remains a need for a vehicle controller to be able to more accurately determine the amount of current required to achieve a given torque output.
A powertrain control unit (PCU) may be configured to apply current to engage the electrical actuator when desired. One example transfers torque from a front wheel drive transaxle to a rear axle of a motor vehicle. Since the amount of current required to transfer each of a range of desired torques will vary, existing PCUs are programmed to use average values. This results in inconsistencies between vehicles when engaging the torque transfer device. As a result, too much or too little torque may be transferred, which can lead to hard or soft engagement of, for example, the rear axle.